1. Field of the Invention
This invention relates to telecommunications systems, and more particularly to a method and apparatus for detecting and determining the point of origin of events in a telecommunications system.
2. Description of Related Art
Public Switched Telephone Networks (PSTN) commonly utilize Time Division Multiplexing (TDM) transmission systems to communicate both voice and data signals over a digital communications link. For example, DS1 (digital signal level 1) data paths are currently used to carry both voice and data signals over a single transmission facility. DS1 paths carry DS1 signals which are transmitted at a nominal rate of 1.544 Mb/s. DS1 paths reduce the number of lines required to carry voice and data signals. Data paths, such as DS1 paths, have a portion of the data transmission capability assigned to communicating customer information from one end of the data path to the other. This portion of the transmission capability is commonly referred to as the “payload”. In addition, another portion of the transmission capability is assigned to overhead functions, such as error detection and maintaining the data path. This portion of the transmission capability is commonly referred to as “overhead”. DS1 facilities are used in large part to carry signals switched by components of the PSTN. However, point-to-point DS1 data links are also used to interconnect equipment controlled by different data users. A typical DS1 signal path is shown in FIG. 1. DS1 transmission systems, like the one shown in FIG. 1, include three general equipment types: (1) terminating equipment, (2) user interface equipment, and (3) transmission equipment. Terminating equipment 10 primarily serves to build the DS1 1.544 Mb/s TDM signal from the various sub-rate voice and data signals. Terminating equipment 10 typically performs Pulse Code Modulation (PCM) and TDM functions. The terminating equipment 10 also de-multiplexes the 1.544 Mb/s DS1 signal to separate voice and data signals at their original sub-rates.
The user interface equipment typically comprises a Channel Service Unit (CSU) 20 which connects the terminating equipment 10 with the transmission equipment 30, such as a DS1 path and ensures that both ends of the DS1 paths 30 send and receive a high quality DS1 signal. The CSU 20 typically checks for conformance to certain standards which are set by the telecommunications industry. The CSU 20 corrects and detects errors in the DS1 transmission path. For example, the CSU 20 corrects Bipolar Violations (BPV). In addition, the CSU 20 detects various errors and inserts alarm indications and zero substitution codes in the DS1 transmission path, including Remote Alarm Indication (RAI), Alarm Indication Signal (AIS), and Bipolar with Eight-Zero Substitution (B8ZS) signals.
The DS1 path 30 includes hardware used by the network providers to transmit DS1 digital signals between equipment controlled by different data users. The DS1 path 30 as shown in FIG. 1 is implemented by a T1 line. However, other facilities, such as coaxial cables, fiber optic cables, and microwave links may be used by providing an appropriate transport interface between the Channel Service Unit (CSU) 20 and the facility.
DS1 signals may be transmitted over a dedicated point-to-point network as simple as the one shown in FIG. 1 utilizing twisted wire pairs and repeaters spaced at intermediate points. Alternatively, the network may be as complex as the one shown in FIG. 2 which utilizes a combination of twisted wire pairs and repeaters, multiplexers, Digital Cross-connect Systems (DCS), Add Drop Multiplexers (ADM), Fiber Optic Terminals (FOT), Coaxial Cable, Microwave, Satellite, or any other transmission media capable of transporting a DS1 signal. In some instances, DS1 signals may be carried over a network similar to the point-to-point network, but having the added capability to switch the DS1 signal (in a DCS or ADM) in a manner similar to the PSTN.
While DS1 transmission systems such as the system shown in FIG. 1 are well-known, customers more typically communicate using a public DS1 network, as shown in FIG. 2. In the DS1 transmission system shown in FIG. 2, equipment is divided into categories based on the location of the equipment. Essentially, the equipment is broken into three categories: (1) the Customer Premises Equipment (CPE) 40; (2) the Local Exchange Carrier (LEC) equipment, which comprises the local loop 42 and the central office equipment; and (3) the InterExchange Carrier (IEC) 52. CPE 40 belongs to the network user (or customer). The customer that owns the CPE 40 is responsible for both its operation and maintenance. The customer must ensure that its equipment provides a healthy and standard DS1 digital signal to the local exchange carrier equipment 42. The equipment 40 on the customer premises typically consists of DS1 multiplexers 46, digital Private Branch Exchanges (PBXs), and any other DS1 terminating equipment which connects to a CSU 20 at the CPE site. The local exchange carrier equipment 42 connects the CPE 40 with the central office 44 and the IEC 52. LECs assume responsibility for maintaining equipment at the line of demarcation between the CPE 40 and the local loop 42.
As shown in FIG. 2, a Network Interface Unit (NIU) 50 may be coupled between the CPE 40 and the LEC equipment 42. The NIU 50 represents the point of demarcation between the CPE 40 and the LEC equipment 42 (which comprises local loop equipment 42, and Central Office (CO) equipment 44). Prior art NIUs may be relatively simple devices which allow network technicians to minimally test the operation and performance of both the CPE 40 and the DS1 network 52 or they may be more sophisticated devices.
The CO equipment 44 may include equipment that can monitor for various DS1 signal requirements. Independent of whether the DS1 transmission system is simple (FIG. 1), complex (FIG. 2), or switched, all the circuits and network equipment required to transmit a DS1 signal must be tested and maintained to operate at maximum efficiency. In order to perform such test and maintenance functions, equipment within the DS1 path provides maintenance signals indicative of particular conditions on the incoming and outgoing signals. These signals are defined by standards established by the American National Standards Institute for operation of DS1 communications links (ANSI T1.403 and ANSI T1.408, et. al). These indications include (1) RAI, which indicates that the signal that was received by the CPE equipment from the NIU 50 was lost (the detailed requirements for sending RAI are contained in ANSI T1.231); and (2) AIS, which is an unframed, all-ones signal which is transmitted to the network interface upon loss, or in response to the presence of a signal defect of the originating signal, or when any action is taken that would cause a service disruption (such as loopback). In addition, detection of signal defects in the digital hierarchy above DS1 shall cause an AIS signal to be generated. The AIS signal is removed when the condition that triggered the AIS is terminated.
In addition to these indications, ANSIT1.403-1995 defines a performance report message (PRM) which is sent each second using a format which is detailed in the standard. PRMs contain performance information within a “message field” portion of the PRM for each of four previous one-second intervals. Counts of cyclic redundancy checking (CRC) errors are accumulated in each contiguous one-second interval by the reporting equipment. For example, a one bit (designated “G1” by the ANSI standard) within the variable portion of the PRM indicates that one CRC error had occurred within the last second of transmission. At the end of each one-second interval, a modulo-4 counter within the variable portion of the PRM is incremented, and the appropriate performance bits are set within the remainder of the variable portion of the report in accordance with the PRM format provided in the specification. Other bits indicate that: (1) between 2 and 5 CRC events had occurred; (2) between 6 and 10 CRC events had occurred; (3) between 11 and 100 CRC events had occurred; (4) between 101 and 319 CRC events had occurred; (5) greater than 319 CRC events had occurred; (6) at least one severely errored framing event had occurred; (7) at least one frame synchronization bit error event had occurred; (8) at least one line code violation event had occurred; and (9) at least one slip event had occurred. The number and type of Events indicates the quality of transmission. Each Event is defined within ANSI T1.403.
Performance reports may be also be generated and transmitted in accordance with AT&T PUB 54016 Performance Reporting. PRMs are only transmitted by equipment that conforms to the Extended Superframe Format (ESF) described in ANSI T1.403. PRMs provide information that can be used in “Sectionalization” of the DS1 path. Sectionalization is a process by which several sections of a data path are analyzed to determine in which section a communication problem originates. In particular, for the purposes of this discussion, sectionalization refers to determining whether a problem in the DS1 path originates within the CPE equipment 40 (for which the customer is responsible), the LEC equipment 42, 44 (for which the local exchange carrier is responsible) or the T1 network equipment 52 (for which the Intermediate Exchange Carrier (IEC) is responsible).
Currently, when a problem is reported on a DS1 path, sectionalization of the path is performed by sending a technician to several points along the path to collect data. The collected data is then reviewed by the technician in an attempt to determine the point of origin of a problem. Since the equipment that makes up the path is physically distributed over a large geographic region (equipment at one end of the path may be thousands of miles from equipment at the other end of the path) it is typical for several technicians to become involved in the sectionalization of the path. Each technician must be highly skilled and trained in order to collect the data from the various points along the path. In some cases, the revenue bearing signals being transmitted over the path must be disrupted in order to perform tests which generate data to be collected or in order to collect data that is developed in real time during the transmission of the revenue generating signals. In many cases the responsibility for sectionalizing the DS1 path must be shared by the LEC, IEC, and customer, since each party is responsible for maintaining a portion of the equipment that makes up the path. Accordingly, each party incurs an expense when equipment maintained by any of the other parties experiences a problem.
Furthermore, out-of-service testing causes “live” traffic to be removed from the DS1 link before testing commences. In out-of-service testing, a test instrument transmits a specific data pattern to a receiving test instrument that anticipates the sequence of the pattern being sent. Any deviation from the anticipated pattern is counted as an error by the receiving test instrument. Out-of-service testing can be conducted on a “point-to-point” basis or by creating a “loop-back”. Point-to-point testing requires two test instruments (a first instrument at one end of the DS1 transmission system, and a second instrument at the other end of the DS1 transmission system). By simultaneously generating a test data pattern and analyzing the received data for errors, the test instruments can analyze the performance of a DS1 link in both directions.
Loop-back testing is often used as a “quick check” of circuit performance or when attempting to isolate faulty equipment. In loop-back testing, a single test instrument sends a “loop-up” code to a loop back device, such as the CSU at the far end, before data is actually transmitted. The loop-up code causes all transmitted data to be looped back by the CSU in the direction toward the test instrument. By analyzing the received data for errors, the test instrument measures the performance of the link up to and including the far end CSU. Because loop-back testing only requires a single test instrument, and thus, only one operator, it is a convenient testing means.
Both point-to-point and loop-back tests allow detailed measurements of any DS1 transmission system. However, because both testing methods require that live, revenue-generating traffic be interrupted, they are undesirable. Thus, out-of-service testing is inherently expensive and undesirable. It is therefore desirable to perform in-service monitoring of “live” data to measure the performance and viability of DS1 transmission systems. Because in-service monitoring does not disrupt the transmission of live, revenue-generating traffic, it is suitable for routine maintenance and it is preferred by both the LECs and their customers.
Referring again to FIG. 2, the prior art NIUs 50 disadvantageously provide only intrusive test and performance monitoring functionality. End-user customers object to the service interruptions and disruptions required by the out-of-service testing performed by the prior art NIUs 50. The LECs install the NIUs 50 at the demarcation point between the CPE 40 and the LEC portions of the network (i.e., at the interface to the local loop 42). The prior art NIUs 50 typically have provided the LECs with a loop-back point for testing DS1 digital circuits to the network boundary. Disadvantageously, customer circuits may be taken out-of-service for intrusive testing only with customer permission. Customers typically do not authorize such intrusive testing means unless a circuit is completely unusable.
There are several types of NIUs 50 currently in use. One of the most popular types of NIUs 50 is the “Smart Jack” available from Westell, Inc., located in Oakbrook, Ill. The Smart Jack NIU with Performance Monitoring (PM) allows the LECs to determine what errors are received and generated by the CPE 40. A major disadvantage of the Smart Jack NIU is that the NIU accumulates PM data and stores the data in a local buffer for later retrieval by LEC personnel. Data retrieval in most areas requires that a circuit be taken completely out-of-service and that the NIU be commanded intrusively using a proprietary command set. Furthermore, the Smart Jack NIU disadvantageously provides no practical method for the LECs to retrieve the performance monitor data collected by the Smart Jack NIU. While the Smart Jack NIU does allow non-intrusive transmission of PM data from the Smart Jack to the central office, a paralleling maintenance line must be provided. Most DS1 installations to customer premises, however, do not provide such maintenance lines.
Other NIUs 50 are available from Wescom Integrated Network Systems (WINS), the Larus Corporation, and Teltrend, Inc. All of the prior art NIUs 50 suffer the disadvantages associated with out-of-service monitoring and testing. Therefore, there is a need for an improved NIU 50 which provides non-intrusive maintenance performance monitoring at the point of demarcation between the CPE 40 and the LEC equipment.
In addition to being unable to provide non-intrusive monitoring of DS1 digital equipment, the prior art NIUs 50 are unable to provide an indication of a loss of signal (LOS) caused by the CPE 40 which is distinguishable from LOSs that are caused by failure of the network equipment. Currently, LOS caused by the CPE 40 generates alarms in the LEC central office equipment 44 which are indistinguishable from the alarms generated in response to LOSs caused by equipment failures in the local loop 42, Central Office 44, or DS1 network 52. Therefore, there is a need for an improved NIU which allows the LECs to distinguish LOS alarm signals caused by loss of signal within the CPE 40 from alarms which originate due to loss of signal within the LEC or network. With such an improved NIU, the LECs can then decide whether to notify their customers of the LOS indication or to ignore the indication as they deem appropriate.
In addition to these disadvantages, the prior art NIUs 50 do not permit the LECs to control the frame format of data transmitted by their customers and transmitted over the LECs' networks. In general, DS1 signals can be transmitted to the local loop 42 using four basic DS1 frame formats: (1) Super Frame format (SF); (2) Extended Superframe Format (ESF) without Performance Report Messages (PRMs); (3) ESF with AT&T PUB 54016 Performance Reporting; and (4) ESF with ANSI T1.403 Performance Report Messages (PRMs). Most DS1 signals are transmitted using the SF format, and the remainder are transmitted by the CPE 40 using a mix of ESF format types. Performance monitoring capabilities of the various formats range from poor in the case of SF (most of the data is not monitored), to excellent, in the case of ESF with ANSI T1.403 PRMs. The difficulty faced by the LECs is that their ability to monitor data and transmission performance is tied to the frame format used by the CPE 40. Because the customer is responsible for the CPE 40, the LECs are unable to control the frame format used and thus the level and extent of performance monitoring and testing that is achievable. The present invention allows the LECs to control the frame format of data by converting the frame format transmitted by the CPE 40.
The ESF format has long been recognized as the single most important change occurring in the telephone network with respect to the quality of service provided on DS1 circuits because it addresses the above-stated need for non-intrusive monitor and test capability. ESF allows customers to continuously and non-intrusively monitor the performance of their DS1 facilities while the applications remain active and thus income-generating. ESF performance monitoring provides both a precise performance report and a proactive maintenance tool. With ESF performance data, a customer can determine correlations between data application performance (response time) and errors which occur on the DS1 facilities. This can aid in troubleshooting end-user response time problems. By looking at the error conditions, the cause of the increased response time can be determined and the appropriate action can be taken.
In addition, the ESF frame format offers the network providers the ability to “sectionalize” problems occurring in the network. By placing ESF monitoring equipment throughout the network, an LEC can monitor the various facilities that make up an end-to-end customer circuit. When customers complain about a degraded or unavailable circuit, the LEC can use the ESF format to locate the faulty link in a real time, non-intrusive manner.
Although the ESF frame format has long been recognized as a tremendous benefit, it has gained little acceptance and use in the CPE 40. Therefore, there is a need for an improved NIU which allows telephone companies to add the ESF functionality to existing DS1 circuits. There is also a need for an improved NIU which will provide telephone companies an adaptive way to increase the number of circuits that use the preferred ESF signal format as the circuit enters the LEC equipment. Moreover, there is a need to combine the functions of network interface, circuit loop-back, frame format conversion, CPE loss of signal detection, and signal degradation detection functionality together in an inexpensive and easily accessible NIU. The present invention provides such an improved NIU.
In addition to the problems which arise due to the inability of CSUs to use ESF frame formatting, it is currently difficult to collect at the Operations System (OS) a sufficient amount of data unobtrusively in real-time to allow automated sectionalization of a data path. Current systems provide for monitoring each data path at the CPE and storing information in the monitoring device until a request is made to communicate the information stored to the OS. However, due to the large amount of information which must be stored, the data is stored in a manner which does not allow the time at which an event occurs to be known with better than a 15 minute resolution. Accordingly, if more than two Events occur in the same 15 minute interval, the ability to distinguish one event from the other is lost. Furthermore, unless there is a reason to suspect a problem on a particular data path, the data is not requested. Still further, in prior art data paths in which data is collected and transmitted at regular intervals (such as by PRMs generated by a CSU) this data is not collected at the network interface, and therefore the ability to sectionalize the data path does not correlate with the portions of the data path which different organizations are responsible for maintaining.
Accordingly, it would be desirable to provide a system which sectionalizes a DS1 path to allow each of the parties responsible for maintaining equipment along the path to determine which party is responsible for the problem. Furthermore, it would be desirable to reduce the cost of sectionalizing a DS1 path by determining the location of the origin of the problem without sending a highly skilled technician to a plurality of locations along the path. Still further, it would be desirable to provide such a system which is capable of sectionalizing a DS1 path in “real-time” without disrupting revenue bearing signals transmitted over the path. The present invention provides such a system.